1. Field
Embodiments of the invention relate to the use of variable delay lines in high-resolution optical coherence tomography.
2. Background
Optical Coherence Tomography (OCT) is a medical imaging technique providing depth resolved information with high axial resolution by means of a broadband light source and an interferometric detection system. It has found plenty of applications, ranging from ophthalmology and cardiology to gynecology and in-vitro high-resolution studies of biological tissues.
One of the elements in a Time Domain OCT (TD-OCT) system is the variable delay line, which may be used to perform the depth scan inside the sample. Several patents have described implementations of delay lines that are able to provide the necessary delay variation range at high scan speeds for their use in OCT. For example, patent application EP 0831312 describes a device based on an optical fiber and a piezoelectric element for its use as a variable delay line in OCT.
However, variable delay line implementations relying on mechanical elements have intrinsic limitations to their maximum operating speed that can be achieved, due to the use of moving parts and their inertia. Recently an implementation of a variable delay line based on integrated optics and taking advantage of silicon's thermo-optical effect has been described (“Thermo-optical delay line for optical coherence tomography” E. Margallo-Balbás, G. Pandraud, and P. J. French, Proc. SPIE 6717, 671704 (2007), “Miniature 10 kHz thermo-optic delay line in silicon” E. Margallo-Balbás, M. Geljon, G. Pandraud, and P. J. French, Opt. Lett. 35 (23). pp. 4027-4029 (2010). These references provide an overview of some advantages of using a thermo-optical delay line.
The thermo-optic effect is based on the variation in phase and group refractive indices of a material with temperature. The relationship between temperature change and refractive index variation is known as the thermo-optic coefficient. As an example, silicon exhibits a value of 2.4×10−4K−1 at room temperature for a wavelength of 1.3 μm, meaning that obtaining a change in optical path of 1 mm requires a temperature increase of 417K for a 1 cm waveguide segment. However, for a given fabrication technology, there is a compromise between the length of the waveguide subject to thermal action, the applied power, the maximum delay (determining the maximum scan depth) and the maximum frequency for thermal cycling (determining scan rate). This trade-off implies that thermal design choices are set once the production process is selected.
One way to relieve the aforementioned trade-off is to trace a waveguide segment several times over an area with a controllable refractive index as described in U.S. patent application publication No. 2009/0022443. Although emphasis is made of good waveguide curvature design to reduce power loss, there is no mention of how to compensate for other optical effects such as birefringence. Birefringence describes an existence of different propagation constants for each polarization mode in a waveguide. (A. Melloni et al., “Determination of Bend Mode Characteristics in Dielectric Waveguides”, J. Lightwave Technol., vol. 19(4), pp. 571-577, 2001).
In many cases, solutions to birefringence are based on the optimization of the waveguide technology itself, such as designing the correct cross-sectional geometry or through the introduction of controlled stress levels to the waveguides. Materials such as thermal silicon oxide have been reported for introducing stress to adjust group and phase velocity of light within the waveguide.
Although these solutions are appropriate in some cases, they complicate the fabrication process and their value depends on tolerances in the deposition and microfabrication steps of the concerned layers. Additionally, they cannot compensate birefringence introduced by waveguide segments having a relatively strong curvature.
Other articles in the literature (“Step-type optical delay line using silica-based planar light-wave circuit (PLC) technology”, I. Kobayashi and K. Koruda, IEEE Instrumentation and Measurement, 1998 and “Wide-bandwidth continuously tunable optical delay line using silicon microring resonators”, J. Cardenas et al., Opt. Express 18, 26525-26534, 2010) report using the thermo-optic effect to produce integrated delay lines. In all cases, however, the application field is different and design parameters diverge significantly from what is required for OCT. Free spectral ranges (FSR) in applications such as telecom are several orders of magnitude smaller than the ones required for OCT. In the first article (Kobayashi et al.) a trade-off between FSR and maximum delay is reported, such that the device would only attain a FSR of approximately 150 GHz in an OCT application. In the second article (J. Cardenas et al.), the corresponding FSR is only 10 GHz. Both ranges are many orders of magnitude away from the ranges used in OCT, which typically utilize bandwidths in the tens of THz's.